Cross-linked olefinic polymers



United States Patent Ofiice 3,075,950 Patented Jan. 29,- 1963 3,075,950 CROSS-LINKED OLEFINIC POLYMERS Gordon C. Newland, Kingsport, Tenn., and James A. Van

Allan, Rochester, N.Y., assignors to Eastman Kodak Company, Rochester, N.Y., a corporation of New Jersey No Drawing. Filed Aug. 4. 1960, Ser. No. 47,391

12 Claims. ((31. 260-66) This invention relates to the preparation of cross-- linked olefinic polymers, and more particularly to crosslinked polyethylenes and shaped articles thereof having improved dimensional stability and improved resistance to swelling and dissolution by organic liquids, by treatment of the olefinic polymeric materials with certain aromatic azides.

It is known that the usefulness of polyethylene shaped articles can be greatly extended by cross-linking treatments, for example, by irradiation. Modified polyethylenes so produced show increased tensile strength, reduced solubility, more abrasion resistance-all desirable changes. However, this method also produces some degradation. Various peroxide treatments of polyethylene, as well as treatment with certain basic organic compounds such as amines, have also been proposed by prior art processes for modifying polyethylenes. For one reason or another, noneof these processes have proven entirely satisfactory for commercial applications.

We have now found a novel and readily reproducible method for preparaing cross-linked polyethylenes, the shaped articles thereof being characterized by greatly improved heat distortion resistance, environmental stresscrackingresistance and resistance to attack and penetration by solvents and swelling agents such as oils and greases as compared with uncross-linked polyethylenes.

These properties are particularly useful in wire coatings,v They are also that are useful tubes, pipes and utensils of all sorts. useful for preparing insolubilized sheets for wrapping purposes and as photographic film supports. The cross-linking, of highly-pigmented polyethylene prod ucts by the process of the invention has a remarkable toughening effect, particularly at low temperatures. Tensile strength at elevated temperatures is increased manyfold. Also, the cross-linked products of the invention are entirely devoid of polyethylene degradation products and oxidized components. In addition, they show great-1y improved adhesion to glass and other surfaces as compared with that shown by peroxide-cross-linked pol ethyl enes or uncross-linked polyethylenes.

It is, accordingly, an object of the invention to provide improved cross-linked polyethylene and related hydrocarbon polymers, and shaped articles thereof, which exhibit improved physical properties. Another object is to provide a novel and unique method for preparing such cross-linked hydrocarbon polymer materials. Other objects will become apparent from a reading of the description and examples of the, invention.

In accordance with the invention, we prepare crosslinked polyethylenes and related hydrocarbon polymers by incorporating one or more specific aromatic azide compounds therein, followed by heating the resulting composition or shaped article until the desired cross-linking or insolubilization has taken place. The proportions can vary over a relatively wide range, for example as little as 0.1% of the aromatic azides of the invention, based on the total weight of the polymer and the azide compound, are effective in producing substantial cross-linking. In general, the more thorough the insolubilization desired, the higher would be the concentration of the azide compound chosen. Practical upper limits of the azide compound concentration are from 2 to 5%, or even more in some cases, depending on the compatibility of the particular azide compound and the polymer, preferably polyethylene, being used. A concentration of from 0.5 to 2.0% of the azide component is particularly efficacious and is the preferred range.

operation by aromatic azides, but thisbehavior" can be minimized by use'o'f lower temperature and longer times of cross-linking treatment. can be used, if desired, to special. advantage in the preparation of cross-linked foamed polyethylenes. The aromatic azide additives can be incorporated into the polyethylenes by any of the known methods of compounding, for example, by mixing the components in powder or granular forms, by spraying the azide onto the polymer powder or granules, by solvent mixing, by' melting the components, extrusion mixing, etc, but preferably by milling on hot rolls. Normally, they are highly polymeric solid, thermoplastic. materials ranging from waxy products to high density solids having relatively high crystallinity as determined by X-ray'diffraction. They are all soluble in hot aromatic hydrocarbon compounds such as tetralin.

The aromatic azide compounds that are particularly suitable in the practice of the invention include 2,2',5,5,'-

tetramethoxy-4,4'-diazido triphenylmethane, 3,-3-dimethoxy-4,4-diazidobiphenyl, 4,4-diazidodibenz'ylidene ace-' tone, 4'-azido-4'-;8-hydroxyethoxy chaleone,- 2,6-di-pazidobenzylidene-4-methylcyclohexanone, 4 ,4'-diazidodi'- phenyl sulfide [H. Bretschnei-de'r et a1., Mo'natsh. 81, pages 970- 1950) and 2,5-bis(4-aZidostyryl)-1,3,4-oxadiazole. Of these, the acetone, the chalcone, the methyl cyclohexanone, the sulfide and the oxadiazole derivatives are outstanding and arethe preferred group.

The following examples serve to illustrate furtherthe preparation of the various intermediate'azide compounds and use thereof in the practice of the invention.

Example 1.-2-Azid0benzoxazole N N I NaNs \0 --ol. .0

Sodium azide (14 g.) in 20 m1. of water was treated with 30.6 g. of 2-chlorobenzoxazole with good shaking.

Yield, 16.0 g.; M.P. 67 C.

Analysis.Calc. for C H N O: C,- 52.4; H, 2 .5. Found: C, 52.6; H, 2.4.

The time and temperature" used in the cross-linking 'heat treatment can also-be varied over fairly widelimits, the. optimum conditions depending onthe particular azide compound selected. Times of 1 minute. to 1 hour, and temperatures of from to 220 6. are the preferred ranges of these variables. Some evolution of gas usually' accompanies the cross-linking- Alternatively, this behavior" a; 6 Example 2.2,2',5,5-Tetrameth0xy-4,4'- Diazidotriplzenylmethane Example 3.--3,3-Dimethoxy4,4'-Diazid0diphenyl 0 CH. (I)GH;

@Q-mn BNO. NaN,

, Dianisidine (24.4 g., 0.1 mole) in 150 ml. of water containing 37 ml. of concentrated hydrochloric acid was treated with 15 g. of sodium nitrite in 50 ml. of water at 0.5 with stirring. After one hour, the diazoniurn solution was filtered from a little insoluble material and a solution of 16 g. of sodium azide in 100 ml. of water was added slowly and with stirring. The diazide, which separated immediately, was filtered oil and crystallized twice from alcohol. Yield 16 g.; M.P. 86 C.

Analysis.-Calc. for C H O N C, 56.7; H, 4.0. Found: C, 56.8; H, 3.7.

Example 4.-4-Azid0-4'- 3-Hydr0xyethoxychalcone noomon=o--coom con-Q no omomo-Q-o o demon-Gar,

a-(p-Hydroxyethoxy)acetophenone (18.0 g.) and 14.7 ml. of p-azidobenzaldehyde in 100 ml. of methanol was treated with 5 ml. of 25 percent sodium hydroxide. After two hours standing, the product was filtered off and dried. Yield, 23 g.; M.P. 120 C.

.Analysis.Calc. for C H O N N, 13.2. Found: N, 13.1.

Example 5.-2,6-Bis(4-Azidobenzylidene)-4- M ethylcy clohexanone ll ans-@0110 O 4-azidobenzaldehyde (16.1 1111., 0.1 mole) and 5.50 ml. (0.05 mole) of 4-methylcyclohexanone in ml. of methanol was treated with 5 m1. of 25 percent caustic. After standing overnight, the product which had precipitated was filtered oil and dried. Yield, 16 g. Crystallization from ethyl acetate gave 14.5 g. of yellow product, M.P. 121 C.

Analysis.-Calc. for CmHmONgl N, 22-7. N, 21.9.

By substituting for the 4-methylcyclohexanone in the above example with an equivalent amount of acetone, the compound 4,4'-diazidobenzylideneacetone having the structure:

was prepared. It had a melting point of 158160 C.

Found Example 6.2,5-Bis(4-Azid0styryl) -1,3,4-Oxadiaz0le NN H.N..H...tl lw.m.m

NP GaGHi LCMa N.

These examples illustrate the eiiicacy of the various aromatic azide compounds of the invention in producing cross-linked polyethylenes. In each case, the polyethylene of density 0.918 and melt index of 2.0 was mixed with the particular aromatic azide additive and hot-roll-compounded for 6 minutes or more. The temperature of the front roll was set at 270 F. and the back roll at 230 F. A sample was then taken of each example composition and compression-molded at 300 F. for 9 minutes to give a plate inch thick. Square sections, 1 inch on the side, were cut from these plates and subjected to crosslinking conditions by heating for 1 hour in a mechanical convection oven at 180 C. Samples of the heated squares cut to /s x A x 1 inch, were tested for solubility by heating 1 hour in tetralin at C. All the compositions of this series of examples were completely soluble in hot tetralin before the cross-linking treatment was applied, i.e., all compositions were processable (rollable or compression-moldable). Accordingly, the degree of resistance to swelling or dissolution by the hot solvent shown by these test strips was taken as a measure of the cross-linking efliciency of the aromatic azide additive which they contained. Cross-linking ratings for the greater adherence to glass surfaces than either peroxideadhesion observed when the compositions of the table additivesweredefined as follows: 5 were heated for one hour on glass plates at 180 C. are

3 Slight Swenino given in the last column. The following. scale of ada 2 moderate swelling heslon was used 1 much swelling 2 strong 0 complete solution 1 moderate However, even at 0 rating somecross-linking is evidenced 10 0 none by increased adhesion to glass. Results for a number of On the whole, correlation between insolubilization and aromatic azides, tested in this way, are recorded in the improvement in adhesion by the efiective cross-linking: following table. Comparison is made therein under the azide additives was very good. Drinking tumblers in-* same test conditions with several peroxides including jection-molded from the compositions of Examples 22, dicumyl peroxide used commercially as a cross-linking 23 and 24, and heated to cause cross-linking, did not disfor polyethylene, and with two aromatic azides which tort when washed with boiling water. On the other hand, proved ineffective cross-linkers, namely, 2-azidobenzoxasimilar untreated (uncross-liked) polyethylene tumblers zole and2,3-diazido-l,4-naphthoquinone [K. Fries et al., distorted badly in boiling water; Ber. 5613, pages 1291-1304 (1923)]. Of these compari- While the above examples in the table are directed to son examples, dicumyl peroxide was the only effective the polyethylene compositions of the invention,'hydrocarcross-linker, but none of them showed any adhesion to bon systems other than polyethylene can also be imglass. It is apparent fromthis table that several of the proved by treatment with the aromatic azides. For ex aromatic azides at a. concentration of only 1% were as ample, articles made of polypropylene wax can be made eifectivein cross-linking polyethylene as dicumyl peroxharder and tougher by preliminary heating of the wax-.- ide at a concentration of 3%. with one or more active azides. In this case, however,

TABLE Free Rad- Concenical Oon- Adhe- Exatuple tration of Cr0sstent of sion to No. Cross Linking Additive Additive, Linking Additive Glass Percent by Rating (No. Per Rating Weight Gram XlQld) None 0 0 Benzoyl peroxide 0 5 0 0 Di-tert-butyl peroxide 0.5 0 0 Tert-butyl hydroperoxide 0. 5 0 0 Cumene hydroperoxide 3. 0 O 0 Metlrylethylketone peroxid 3.0 0 0 2,5-Dimethylhexane-2,5-dihyd ps1 3.0 0 0 Dicumyl peroxide 3.0 3 0 a,u-Diphenyl-Bpicryl hydrazyl... 1. 0 0 0 2Azido benzoxazole 1. 0 0 0 2,3-DiazidodA-naphthoquinone 1. 0 0 0 2,2,5,5-Tetramethoxy-4,4-diazido triphenylmethane 1.0 0 1 3,3-Dimethoxy-t,4-diazidobiphenyl 1. 0 0 2 4,4-Diazidodibenzylidene acetone. 1, 0 2 1 4-Azid0-4-fl-hydroxyethoxy chalcone. 1. 0 2 2 2, 6-D i-p-azidobenzylidene-4-methylcyclohexanone 1. 0 3 2 4,4-Diazidodiphenyl Sulfide 1.0 3 2 2,5-Bis (4-azidostyryl)-1,3,4-oxadiazole 1. 0 3 2 It will be noted from the table that aside from the dino insolubilization of the product was noted. The crosscumyl peroxide cross-linking was obtained only with the linking must have remained in the incipient stage. Simiazides showing measurable free radical contents (eleclar results were obtained with various asph-alts to give tron paramagnetic resonance spectra measurements exproducts having improved hardness, viscosity and surpressed as the number of free radicals per gram). Howface-cracking resistance. ever, this property cannot be used as the measure of The invention has been described in detail with parcross-linking for compounds other than the aromatic ticular reference to preferred embodiments thereof, but azides because Example 15 of the table shows the wellit will be understood that variations and modifications can known stable free radical, u,a-diphenyl- 8-picryl hybe efiected within the spirit and scope of the invention drazyl, to have neither cross-linking activity nor adhesion as described hereinabove and as defined in the appended to glass. The very effective cross-linker, 2,5-bis-(4 claims. azidostyryl)-l,3,4-oxadiazole, which gave a weak free What we claim is: radical signal per se, gave a strong signal after being 1. A process for modifying a solid, high molecular compounded into the polyethylene. The ineifective weight polyolefin selected from the group consisting of cross-linker, 3,3'-dimethoxy 4,4 diazobiphenyl, which polyethylene and polypropylene which comprises heating gave a strong free radical signal per se, gave a much a mixture comprising from 98.0 to 99.5% by weight of lower signal after compounding with the polyethylene. the said polyolefin and conversely from 2.0 to 0.5% by The inefiective cross-linkers, 2,3-diazido-1,4-naphthoweight of an aromatic azide containing only carbon, hyquinone and 2-azido-benzoxazo-le, developed no measuradrogen, oxygen and nitrogen atoms and wherein each ble signals on compounding. In other words, the aroazide group is attached directly to a ring carbon atom matic azide compound must contain some measurable having a free radical content greater than 10 per gram, free radical content, i.e. contain more than 10 free radiat a temperature of from 170 to 220 C., the amount cals per gram, to be efficacious as a cross-linker for polyof said aromatic azide being sufficient to modify said ethylene. Also, it may be noted that the polyethylene polyolefin. compositions cross-linked by aromatic azides had much 2. A process for modifying a solid, high molecular weight polyethylene which comprises heating a mixture comprising from 98.0 to 99.5 by weight of said polyethylene and conversely from 2.0 to 0.5% by weight of 4,4'-diazidodibenzylidene acetone, at a temperature of from 170 to 220 C.

3. A process for modifying a solid, high molecular weight polyethylene which comprises heating a mixture comprising from 98.0 to 99.5% by weight of said polyethylene and conversely from 2.0 to 0.5 by weight of 4-azido-4-p-hydroxyethoxy chalcone, at a temperature of from 170 to 220 C.

4. A process for modifying a solid, high molecular weight polyethylene which comprises heating a mixture comprising from 98.0 to 99.5% by weight of said polyethylene and conversely from 2.0 to 0.5% by weight of 2,6-di-p-azidobenzylidene 4 methylcyclohexanone, at a temperature of from 170 to 220 C.

5. A process for modifying a solid, high molecular weight polyethylene which comprises heating a mixture comprising from 98.0 to 99.5% by weight of said polyethylene and conversely from 2.0 to 0.5 by weight of 4,4'-diazidodiphenyl sulfide, at a temperature of from 170 to 220 C.

6. A process for modifying a solid, high molecular weight polyethylene which comprises heating a mixture comprising from 98.0 to 99.5% by weight of said polyethylene and conversely from 2.0 to 0.5 by weight of 2,5-bis(4-azidostyryl)-l,3,4-oxadiazole, at a temperature of from 170 to 220 C.

7. A modified polyolefin obtained according to the process of claim 1.

8. A modified polyethylene obtained according to the process of claim 2.

9. A modified polyethylene obtained according to the process of claim 3.

10. A modified polyethylene obtained according to the process of claim 4.

11. A modified polyethylene obtained according to the process of claim 5.

12. A modified polyethylene obtained according to the procss of claim 6.

References ited in the file of this patent UNITED STATES PATENTS 2,692,826 Neugebauer et -al Oct. 26, 1954 2,764,599 Clifiord et al Sept. 25, 1956 2,830,978 Muller et a1. Apr. 15, 1958 2,940,853 Sagura et a1 June 14, 1960 

5. A PROCESS FOR MODIFYING A SOLID, HIGH MOLECULAR WEIGHT POLYETHYLENE WHICH COMPRISES HEATING A MIXTURE COMPRISING FROM 98.0 TO 99.5% BY WEIGHT OF SAID POLYETHYLENE AND CONVERSELY FROM 2.0 TO 0.5% BY WEIGHT OF 4.4''-DIAZIDODIPHENYL SULFIDE, AT A TEMPERATURE OF FROM 170* TO 220*C. 